Method for coding a picture sequence, corresponding method for reconstruction and stream of coded data representative of said sequence

ABSTRACT

A method for reconstruction of a picture sequence coded in accordance with a coding method specifying a set of coding tools and/or their associated coding parameters is disclosed. The pictures being divided into coding entities. The method for reconstruction according to the invention comprises the following steps for each coding entity coded in INTER mode:
         determining for the coding entity at least one reference picture, and   reconstructing the coding entity from the at least one reference picture with coding tools configured by coding parameters associated with the coding tools.       

     Advantageously, the coding tools and/or the associated coding parameters depend on the reference picture.

1. SCOPE OF THE INVENTION

The invention relates to the general domain of picture coding.

The invention relates to a method for coding a picture sequence, acorresponding method for reconstruction and a stream of coded datarepresentative of said picture sequence.

2. PRIOR ART

It is known in the art to code a current picture from a picture sequenceby temporal prediction from other pictures of the sequence, known asreference pictures, previously coded and reconstructed. These referencepictures are stored in a buffer.

It is also known in the art to store in such a buffer different versionsof a same reference picture, for example one or more filtered versionsof this reference picture.

The coding of a current picture implements different coding tools, forexample coding modes, quantization, entropy coding, motion compensationwhich are configured by various parameters. For example, the motioncompensation is configured by the precision of motion vectors, thequantization is for example configured by the quantization type that canbe uniform or logarithmic.

During the coding of a current picture, respectively during itsreconstruction, the coding configuration implemented, that is to say thecoding tools and/or the associated coding parameters, are specifiedaccording to the current picture to be coded respectively to bereconstructed. For example, the video coding standard H.264/MPEG-4 AVCspecifies the coding modes of blocks of a picture according to the typeof this current picture. If the current picture is of type I then onlythe spatial prediction modes (INTRA modes) are authorized, if thecurrent picture is of type P then the mono-directional temporalprediction modes (INTER modes) are authorized in addition to INTRAmodes.

The specialisation according to the current picture type to be coded isefficient as it avoids using coding configurations unsuited to thedifferent picture types considered. However, it may be that within apicture or a picture zone, some configurations are used more often andothers are used rarely. This requires an extra signalling cost and thusa loss in coding efficiency as even rarely used configurations must beable to be signalled.

3. SUMMARY OF THE INVENTION

The purpose of the invention is to overcome at least one of thedisadvantages of the prior art. For this purpose, the invention relatesto a method for reconstruction of a picture sequence coded in accordancewith a coding method specifying a set of coding tools and/or theirassociated coding parameters, the pictures being divided into codingentities. The method for reconstruction according to the inventioncomprises the following steps for each coding entity coded in INTERmode:

-   -   determining for the coding entity at least one reference        picture, and    -   reconstructing the coding entity from the at least one reference        picture with coding tools configured by coding parameters        associated with the coding tools.        Advantageously, the coding tools and/or the associated coding        parameters depend on the reference picture.

According to a particularly advantageous aspect of the invention, themethod for reconstruction also comprises a step for decoding a profileidentifier associated with the at least one reference picture, theprofile defining a set of coding tools and/or associated codingparameters.

According to a particular aspect of the invention, the profileidentifier is decoded from a header of the picture to which the codingentity belongs.

According to another particular aspect of the invention, the profileidentifier is decoded from a header of the picture slice to which thecoding entity belongs.

According to a particular embodiment, the method for reconstruction alsocomprises a step of decoding of a number of profiles and, for eachprofile, of data specifying a set of coding tools and/or codingparameters associated with the tools.

Advantageously the step of reconstruction of the coding entity comprisesthe following steps:

-   -   determining for the coding entity a prediction coding entity by        motion compensation of the at least one reference picture,    -   decoding for the coding entity a coefficients coding entity,    -   dequantizing the coefficients coding entity into a dequantized        coefficients coding entity,    -   transforming the dequantized coefficients coding entity into a        residues coding entity, and    -   merging the prediction coding entity and the residues coding        entity to form the reconstructed coding entity.

According to a particular aspect of the invention, when the referencepicture is a picture filtered with a low-pass filter, then the codingtools used to reconstruct the coding entity are a subset of the set ofcoding tools and/or associated coding parameters specified by the codingmethod.

Advantageously, the subset comprises:

-   -   a motion compensation with bilinear interpolation filter and        with motion vectors of ½ pixel precision,    -   a uniform quantization with a quantization matrix favouring low        frequencies,    -   an entropy coding with scanning of transformed coefficients        adapted to the direction of the low-pass filtering of the        reference picture.

According to another particular aspect of the invention, when thereference picture is a picture filtered with a filter improving itsresolution, then the coding tools and/or associated coding parametersused to reconstruct the coding entity comprise a motion compensationwith a polyphase linear interpolation filter and with motion vectors of⅛ pixel precision.

According to another particular aspect of the invention, when thereference picture is a motion compensated picture according to a modelof global motion, then the coding tools used to reconstruct the codingentity comprise the following coding tools and/or associated codingparameters:

-   -   motion compensation with bilinear interpolation filter with        motion vectors of ⅛ pixel precision having a range limited to N,    -   a partitioning limited to blocks of size 16×16, and    -   a favoured skip mode.

According to a particular characteristic of the invention, N=2.

The invention also relates to a stream of coded data representative of apicture sequence divided into coding entities comprising, in a headerassociated with at least a part of the picture sequence, an item of dataspecifying a number of profiles and for each profile data defining a setof coding tools and/or their associated coding parameters and comprisingfor each picture of the sequence in a header associated with at least apart of the picture, an identifier indicating, for each referencepicture used to code the coding entities of the picture part, anassociated profile.

The invention also relates to a method for coding a picture sequence,the pictures being divided into coding entities. The method for codingcomprises the following steps for each coding entity coded in INTERmode:

-   -   determining for the coding entity at least one reference        picture, and    -   coding the coding entity from the at least one reference picture        with coding tools configured by coding parameters associated        with the coding tools. Advantageously, the coding tools and/or        the associated coding parameters depend on the reference        picture.

4. LIST OF FIGURES

The invention will be better understood and illustrated by means ofembodiments and advantageous implementations, by no means limiting, withreference to the figures in the appendix, wherein:

FIG. 1 shows a method for reconstruction according to the invention,

FIG. 2 shows a coding method according to the invention,

FIG. 3 shows a decoding device according to the invention, and

FIG. 4 shows a coding device according to the invention.

5. DETAILED DESCRIPTION OF THE INVENTION

A picture comprises pixels or picture points with each of which areassociated at least one item of picture data. An item of picture data isfor example an item of luminance data or an item of chrominance data.

The term “coding entity” designates the basic structure of a picture tobe coded or reconstructed. It comprises a subset of pixels of thepicture. The set of coding entities of a picture can have or not havecommon pixels. Generally, the coding entity is a block of pixels.However, the term “coding entity” is generic and can designate a circle,a polygon or a region of any shape. Hereafter the term block is used tosimplify the description. However, throughout the description this termcan be replaced by the generic term of coding entity.

The term “residue” or “prediction residue” designates the data obtainedafter extraction of other data. The extraction is generally asubtraction of prediction pixels from source pixels. However, theextraction is more general and comprises notably a weighted subtraction.

The term “reconstructs” designates data (for example pixels, codingentities, blocks) obtained after merging of residues with predictiondata. The merge is generally a sum of prediction pixels with residues.However, merging is more general and comprises notably the weighted sum.A reconstructed coding entity is a coding entity comprisingreconstructed pixels.

In reference to picture decoding, the terms “reconstruction” and“decoding” are very often used as being synonymous. Thus, a“reconstructed block” is also designated under the terminology of“decoded block”.

A picture sequence is generally formed of GOPs (Group Of Picture). Eachpicture is formed of blocks. Each block generally belongs to a pictureslice, which is thus formed of several blocks.

A reference picture used to predict another picture is identified in thesequence using a reference picture index.

According to the invention a reference pictures buffer contains picturesthat correspond to different picture types. These pictures can notablycorrespond to:

-   -   “source type” pictures, that is to say pictures previously        reconstructed, corresponding to distinct temporal instants,    -   “filtered type” pictures, that is to say filtered versions of        pictures previously reconstructed, for example for which the        spatial definition is reduced,    -   “compensated type” pictures, that is to say motion compensated        versions of previously reconstructed pictures, for example        reference pictures compensated by a global motion,    -   etc.

The invention relates to a method for reconstruction described inreference to FIG. 1 and a corresponding method for coding described inreference to FIG. 2. According to the invention, the method for codingrespectively for reconstruction specializes the process for codingrespectively for reconstruction of a current block in INTER mode, i.e.temporally predicted, according to the reference picture used to predictthis current block in such way that the process in question dynamicallyadapts according to the reference pictures used. The coding andreconstruction configurations adapt by modifying the coding tools and/ortheir associated coding parameters used to code the current block. Theexample of the following tools can be cited:

-   -   INTER type coding modes, for example P mode (mono-directional)        pointing to a single reference picture, B mode (bi-directional)        pointing to two reference pictures, skip mode that does not        require the coding of any motion or residue information,    -   partitioning type that describes a partition of the block, the        H.264/MPEG-4 AVC video coding standard defines, for example, for        a macroblock the following partitions: 16×16, 16×8, 8×16, 8×8,        the 8×8 partitions can themselves be sliced into sub-partitions        8×4, 4×8, 4×4,    -   motion compensation type (for example motion compensation with a        bilinear, bi-cubic, de Lanczos, etc. filter) configured by a        precision of motion vectors, possibly by a maximum amplitude of        these vectors, possibly by the size and the coefficients of        interpolation filters,    -   quantization type (uniform, logarithmic, etc.) configured by a        quantization step, possibly by a quantization step offset        (offset with respect to the step attributed to the picture),        possibly by a quantization matrix, possibly by the size of the        “dead-zone”,    -   transform type of which several versions can be specified in the        coding/reconstruction methods (for example Discrete Cosine        Transform, Discrete Wavelet Transform, Discrete Sine Transform,        Karhunen-Loeve Transform, etc) configured for example by the        size of the transform, possibly by the value of the transform        coefficients,    -   entropy coding/decoding type (for example Variable Length        Coding, Context-Adaptive Binary Arithmetic Coding, Arithmetic        Coding) configured notably by a type of scanning type of        transform coefficients, in the case of the H.264/MPEG-4 AVC        standard, the CABAC (Context-Adaptive Binary Arithmetic Coding)        coding is configured notably by contexts, initial probabilities        and a probabilities convergence speed.

According to the invention, different profiles are defined accordingnotably to the reference picture types defined above. Each is thereforeassociated with a reference picture type. Each profile defines a set ofcoding tools and/or their associated coding parameters. As an examplethe following profiles are defined:

-   -   A generic profile that applies to the “source type” reference        pictures and that comprises the standard tools (for example the        coding tools specified by the H.264/MPEG-4 AVC standard) with        their associated coding parameters.    -   A low resolution profile that applies to the “filtered type”        reference pictures, with low-pass filters, this filter can be        isotropic or directional if there is a desire to give priority        to a filtering direction. This profile is a subset of the        generic profile particularly adapted to the case of “filtered        type” reference pictures. This profile comprises the following        tools with their associated coding parameters:        -   motion compensation with a bilinear interpolation filter and            with motion vectors of ½ pixel precision, the signal being            more “flat”, it is of no use to call upon complicated            interpolation filters, for the same reason, the motion does            not need to be very precise; even more so if the filter            applied to generate the reference picture is directional            (blurring according to an orientation), the precision of the            motion according to this orientation can be reduced though            for the perpendicular orientation, the original precision is            conserved,        -   uniform quantization with a quantization matrix favouring            low frequencies, in fact if it is used as a reference            picture, a low-pass filtered picture, this means that a flat            texture is favoured thus having few high frequencies, it is            thus preferable to favour coding at these low frequencies,        -   entropy coding, whatever the type (that is to say in this            case only the coding parameters are specified), with            scanning of transformed coefficients adapted to the            direction of the low-pass filtering if this filtering is            anisotropic, typically, if the low-pass filtering of the            reference picture is implemented only according to the            horizontal direction, the transformed coefficients will            rather present high frequencies according to the vertical            axis, it would thus be preferable to give priority to a            scanning according to this axis of transformed coefficients,        -   The other coding tools and/or the associated coding            parameters of the low resolution profile correspond to those            of the generic profile.    -   A high resolution profile that applies to the “filtered type”        reference pictures, with a filter improving its resolution,        enabling for example to further increase the contrast, the        contours of the reference picture. This profile comprises the        following tools with their associated coding parameters:        -   Motion compensation with polyphase linear interpolation            filters (the number of phases being defined by the motion            precision) and with motion vectors of ⅛ pixel precision,        -   The other coding tools and/or the associated coding            parameters of the high resolution profile correspond to            those of the generic profile.    -   A Global Motion Compensation (GMC) profile that applies to the        motion “compensated type” reference pictures on the basis of        global motion models. This profile comprises the following tools        with their associated coding parameters:        -   motion compensation with a bilinear interpolation filter,            the motion compensation already carried out already enables            the signal to be interpreted precisely therefore a simpler            interpolation is a priori sufficient, with low range of            motion vectors (typically 2 pixels), in fact the reference            picture being already motion compensated, the coded motion            vector only serves to correct a motion compensation residual            error, and with motion vectors of a precision of ⅛ pixel, as            the correction of a motion residual error must be able to be            precise,        -   16×16 partitioning only as the global motion compensation is            applied over large zones of the picture,        -   Skip mode favoured, that is to say more priority is given to            this mode than the other possible modes,        -   The other coding tools and/or the associated coding            parameters of the GMC profile correspond to those of the            generic profile.

According to other variants, ad'hoc profiles can also be defined.

Ad'hoc profiles are dynamically defined by the coding method followinguse analysis of reference pictures. For example, during a first codingpass, the set of tools and authorized configurations are used for allthe reference pictures. An analysis of tools used according to referencepicture type is then carried out. If this analysis demonstrates thattools and/or tool configurations are more specifically used on somereference pictures, adapted profiles can be defined and signalled forthese reference pictures. During a second coding pass, the definedprofiles are used which augments the coding efficiency due to areduction in signalling costs. For example, it may appear during a firstcoding pass that on some reference pictures a given motion compensationtype (for example with bilinear filter) is retained in the majority ofcases. This motion compensation type is then given priority in theprofile of this reference picture for the second coding pass. Accordingto a second example the skip mode is specialised according to thereference picture used. If, for a reference picture, a given codingconfiguration is used in the majority of cases (for example 16×16partitioning with AVC interpolation filter and motion vectors of ½ pixelprecision) and if for this majority configuration, the predictionresidue to be coded is statistically often null, then the skip mode willbe associated with this majority configuration, that is to say that ablock coded in skip mode is reconstructed by motion compensation of a16×16 block with an AVC interpolation filter and ½ pixel motionprecision.

FIG. 1 shows a method for reconstruction of a current block Bc of acurrent picture Ic of a video sequence presented in the form of a streamF of coded data. The current block also belongs to a current slice Sc.The method for reconstruction is preferentially implemented on a videodecoder as shown in FIG. 3.

During a step 10, a coding mode DEC_MODE is decoded for the currentblock from the stream F.

During a step 12, an index or indexes DEC_IDX of reference pictures fromwhich the current block is predicted is/are decoded for the currentblock from the stream F when the coding mode of the current block is anINTER mode. According to a variant of this step, the index or indexes ofreference pictures from which the current block is predicted is/aredetermined for the current block from the index or indexes of referencepictures associated with blocks reconstructed previous to the currentblock that are spatially neighbouring the current block or from theco-located block of the last reconstructed picture.

During a step 14, the current block is reconstructed using coding toolsand/or their associated coding parameters determined according to thereference picture or pictures identified by their indexes determined instep 12. For this purpose, for each reference picture used toreconstruct current picture blocks Ic a profile identifier is decodedfrom the stream F, for example from the header of the picture Ic.According to a variant, for each reference picture used to reconstructcurrent slice blocks Sc a profile identifier is decoded from the streamF, for example from the header of the slice Sc. Each profile defines aset of coding tools and/or their associated coding parameters. Theseprofiles can for example be specified in the header of the picturesequence or in the header of a group of pictures. Some among them can bedefined by default and thus do not have to be in the stream F. Thus,with the indexes of the reference pictures decoded in step 12, thereference pictures of the buffer required for its reconstruction aredetermined for the current block Bc. With the decoded profileidentifiers of the header Ic or Sc, the associated profile and thus thecoding tools and/or their associated coding parameters to reconstructthe current block Bc are determined for each reference picture used. Inthe case where the block is predicted in bi-prediction mode from 2reference pictures using 2 different profiles, three cases are to beconsidered:

-   -   1. The tools and/or associated coding parameters of two profiles        are identical in which case, they can be applied as is,    -   2. The tools and/or associated coding parameters of two profiles        are not concurrent and apply independently on each        mono-directional prediction of the bi-prediction mode (for        example, the motion compensation applies independently for each        mono-directional prediction, as a consequence the interpolation        filters and the motion vector precisions for each prediction do        not concur), in this case the tools and coding parameters are        applied as specified in the profiles, independently for each        mono-directional prediction,    -   3. The tools and/or associated coding parameters specified in        each profile are directly concurrent (for example the        quantization of prediction residues), in this case the        configuration specified in the generic profile is used.

The block Bc is then reconstructed in the standard manner by merging ofa temporal prediction block and a residues block. The residues block isobtained by decoding a part of the stream F into a block of coefficientson which is then applied an inverse quantization and possibly an inversetransform.

FIG. 2 shows a method for coding a current block Bc of a current pictureof a video sequence presented in the form of a stream F of coded data.The current block also belongs to a current slice Sc. The method forcoding is preferentially implemented on a video coder such as that shownin FIG. 4.

During a step 20, a coding mode DET_MODE is determined for the currentblock.

During a step 22, an index or indexes DET_IDX of reference pictures fromwhich the current block is predicted is/are determined for the currentblock when the coding mode of the current block is an INTER mode.According to a variant of this step, the index or indexes of referencepictures from which the current block is predicted is/are determined forthe current block from the index or indexes of reference picturesassociated with blocks coded previous to the current block that arespatially neighbouring the current block or from the co-located block ofthe last coded picture.

During a step 24, the current block is coded using coding tools and/ortheir associated coding parameters determined according to the referencepicture or pictures identified by their indexes determined in step 22.

With the indexes of reference pictures determined in step 22, thereference pictures of the buffer required for its coding are determinedfor the current block Bc. According to the type of reference picturesthus determined, the associated profile(s) is/are deduced. For example,if the current block B is predicted from a “filtered” type referencepicture with a low-pass filter then the low resolution profile is used.The block Bc is then coded in the standard way by extracting from thecurrent block a temporal prediction block in order to obtain a residuesblock. The prediction block is obtained by motion compensation of thereference picture with motion vectors. The residues block is thentransformed, quantized and coded in the stream F. The fact of using alow resolution profile implicates notably during the motion compensationusing motion vectors with a ½ pixel precision and a bilinearinterpolation filter.

A profile identifier is also coded in the stream F, for example in theheader of the picture Ic, and this for each reference picture used tocode the blocks of the current picture Ic. According to a variant, aprofile identifier is coded in the stream F, for example in the headerof the slice Sc, for each reference picture used to code blocks of thecurrent slice Sc.

Each profile defines a set of coding tools and/or their associatedcoding parameters. These profiles can for example be specified in theheader of the picture sequence or in the header of a group of pictures.Some among them can be defined by default and do not thus have to be inthe stream F. With the profile identifiers coded in the header of Ic orof Sc, the associated profile and thus the coding tools and/or theirassociated coding parameters used to code the current block Bc can thusbe determined for each reference picture used.

The invention also relates to a stream of coded data or coded datastructure representative of a picture sequence comprising in a headerassociated with the sequence or a part of this sequence, that is to saywith a group of pictures, an item of data defining a number of profilesand comprising for each profile data defining a set of coding toolsand/or their associated coding parameters. The stream of coded data alsocomprises in the header of each picture Ic or each picture slice Sc anidentifier indicating, for each reference picture used to code theblocks of this picture Ic respectively of this slice Sc, the profilethat is associated with it.

FIG. 3 diagrammatically shows a decoding device 13. The decoding device13 receives at input a stream F representative of a picture. The streamF is for example transmitted by a coding device 12 via a channel. Thedecoding device 13 is able to implement the reconstruction methodaccording to the invention described in reference to FIG. 1. Thedecoding device 13 comprises an entropy decoding module 1300 able togenerate decoded data. The decoded data are then transmitted to a module1302 able to carry out an inverse quantization IQ followed by an inversetransform IT. The module 1302 is connected to a calculation module 1304able to merge the block from the module 1302 and a prediction block Bpto generate a reconstructed current block Bc that is stored in a memory1306. The decoding device 13 also comprises a motion compensation module1308. The motion compensation module 1308 determines a prediction blockBp from picture data already reconstructed stored in the memory 1306 andfrom decoded motion data transmitted by the entropy decoding module1300. The modules of the decoding device 1300, 1302 and 1308 use codingtools and/or associated coding parameters determined according to step14 of the method for reconstruction.

FIG. 4 diagrammatically shows a coding device 12. The coding device 12receives at input a picture or pictures. The coding device 12 is able toimplement the coding method according to the invention described inreference to FIG. 2. Each picture is divided into blocks of pixels witheach of which is associated at least one item of picture data. Thecoding device 12 notably implements a coding with temporal prediction.Only the modules of the coding device 12 relating to the coding bytemporal prediction or INTER coding are shown in FIG. 4. Other modulesknown by those skilled in the art of video coders are not shown (forexample selection of the coding mode, spatial prediction). The codingdevice 12 notably comprises a calculation module 1200 able to extractfrom a current block Bc a prediction block Bp to generate a residueblock Br. It further comprises a module 1202 able to transform thenquantize the residue block Br into quantized data. The coding device 12also comprises an entropy coding module 1204 able to code quantized datain a stream F. It also comprises a module 1206 carrying out the inverseoperation of the module 1202. The module 1206 is identical to the module1302 of the decoding device 13. The module 1206 carries out an inversequantization IQ followed by an inverse transform IT. The module 1206 isconnected to a calculation module 1208 able to merge the block of datafrom the module 1206 and the prediction block Bp to generate areconstructed block that is stored in a memory 1210. A motioncompensation module 1216 determines the prediction block Bp from picturedata already reconstructed stored in the memory 1210 and from motiondata (i.e. motion vectors and reference picture indexes) determined by amotion estimation module 1212. The modules of the coding device 1202,1204, 1206 and 1216 use coding tools and/or associated coding parametersdetermined according to step 24 of the coding method.

The tools and coding parameters mentioned previously are not exhaustive.Other tools and coding parameters can be considered and used in thescope of the invention. For example, the prediction type of the motionvector can be cited (for example the “Template Matching” predictor orthe median of neighbouring vectors, or the vector from the co-locatedblock, etc.) configured for example by a number of predictors

1. A method for reconstruction of a picture sequence coded in accordancewith a method for coding specifying a set of coding tools and/or theirassociated coding parameters, said pictures being divided into codingentities comprising the following steps for each coding entity coded inINTER mode: determining for said coding entity at least one referencepicture, reconstructing said coding entity from the at least onereference picture with coding tools configured by coding parametersassociated with said coding tools, wherein said coding tools and/or saidassociated coding parameters depend on said reference picture type.
 2. Amethod for reconstruction according to claim 1, further comprising thestep of decoding, in a header associated with at least a part of saidpicture sequence, of an item of data specifying a number of profiles,each profile being associated with a reference picture type, and, foreach profile, data defining a set of coding tools and/or theirassociated coding parameters.
 3. A method for reconstruction accordingto claim 2, further comprising a step of decoding, in a headerassociated with at least a part of the picture to which belongs saidcoding entity, of a profile identifier associated with the at least onereference picture indicating an associated profile.
 4. A method forreconstruction according to claim 3, wherein said profile identifier isdecoded from a picture slice header to which belongs said coding entity.5. A method for reconstruction according claim 1, wherein said referencepicture type belongs to the set of picture type: source picture;filtered picture; and motion compensated picture.
 6. A method forreconstruction according to claim 1, wherein the step of reconstructionof said coding entity comprises the following steps: determining forsaid coding entity a prediction coding entity by motion compensation ofthe at least one reference picture, decoding for said coding entity acoding entity of coefficients, dequantizing said coefficients codingentity into a dequantized coefficients coding entity, transforming saiddequantized coefficients coding entity into a residues coding entity,and merging said prediction coding entity and said residues codingentity to form said reconstructed coding entity.
 7. A method forreconstruction according to claim 1, wherein when the reference pictureis a picture filtered with a low-pass filter, then said coding toolsused to reconstruct said coding entity are a subset of the set of codingtools and/or associated coding parameters specified by said codingmethod.
 8. A method for reconstruction according to claim 7, whereinsaid subset comprises: a motion compensation with bilinear interpolationfilter and with motion vectors of ½ pixel precision, a uniformquantization with quantization matrix favouring low frequencies, anentropy coding with scanning of transformed coefficients adapted to thedirection of the low-pass filtering of said reference picture.
 9. Amethod for reconstruction according to claim 1, wherein when thereference picture is a picture filtered with a filter improving itsresolution, then said coding tools and/or associated coding parametersused to reconstruct said coding entity comprise a motion compensationwith a polyphase linear interpolation filter and with motion vectors of⅛ pixel precision.
 10. A method for reconstruction according to claim 1,wherein when the reference picture is a motion compensated pictureaccording to a model of global motion, then said coding tools used toreconstruct said coding entity comprise the following coding toolsand/or associated coding parameters: motion compensation with bilinearinterpolation filter with motion vectors of ⅛ pixel precision having arange limited to N, a partitioning limited to blocks of size 16×16, andmore priority given to skip mode.
 11. A method for reconstructionaccording to claim 10, wherein N=2.
 12. A stream of coded datarepresentative of a picture sequence divided into coding entitiescomprising, in a header associated with at least a part of said picturesequence, an item of data specifying a number of profiles, each profilebeing associated with a reference picture type, and, for each profile,comprising data defining a set of coding tools and/or their associatedcoding parameters and comprising, for each picture of said sequence, ina header associated with at least a part of said picture, a profileidentifier indicating, for each reference picture used to code thecoding entities of said picture part, an associated profile.
 13. Amethod for coding a picture sequence, said pictures being divided intocoding entities comprising the following steps for each coding entitycoded in INTER mode: determining for said coding entity at least onereference picture, coding said coding entity from the at least onereference picture with coding tools configured by coding parametersassociated with said coding tools, wherein said coding tools and/or saidassociated coding parameters depend on said least one reference picturetype.